/*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
    *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.giraph.block_app.reducers;
  
  import org.apache.giraph.reducers.impl.KryoWrappedReduceOperation;
  
  import java.io.DataInput;
  import java.io.DataOutput;
  import java.io.IOException;
  import java.util.PriorityQueue;
  
  /**
   * Extracts top N largest elements
   *
   * @param <S> Single value type, objects passed on workers
   */
  public class TopNReduce<S extends Comparable<S>>
    extends KryoWrappedReduceOperation<S, PriorityQueue<S>> {
    private int capacity;
  
    public TopNReduce(int capacity) {
      this.capacity = capacity;
    }
  
    public TopNReduce() { }
  
    @Override
    public PriorityQueue<S> createValue() {
      return new PriorityQueue<S>();
    }
  
    @Override
    public void reduce(PriorityQueue<S> heap, S value) {
      if (capacity == 0) {
        return;
      }
  
      if (heap.size() < capacity) {
        heap.add(value);
      } else {
        S head = heap.peek();
        if (head.compareTo(value) < 0) {
          heap.poll();
          heap.add(value);
        }
      }
    }
  
    @Override
    public void reduceMerge(
      PriorityQueue<S> reduceInto,
      PriorityQueue<S> toReduce
    ) {
      for (S element : toReduce) {
        reduce(reduceInto, element);
      }
    }
  
    @Override
    public void write(DataOutput out) throws IOException {
      super.write(out);
      out.writeInt(capacity);
    }
  
    @Override
    public void readFields(DataInput in) throws IOException {
      super.readFields(in);
      capacity = in.readInt();
    }
  }